Fibre optic transmitters for transmitting serial data streams frequently include LED (light emitting diode) driver circuits controlled by output pads of integrated circuits. Due to the higher currents conducted by the driver circuits, they become the most susceptible to failure. Performance monitoring of the driver circuits themselves has not been performed in conventional designs because propagation delays vary so widely, usually being longer than one system clock cycle. The driver circuits introduce delay as do output pads, and propagation paths. The propagation delay will also vary depending upon temperature, voltage and process. Fault isolation has been typically performed by looking for bad data at the receiver. However, when a fault is detected, it is not possible to isolate the fault to the transmitter or the receiver or the fibre link itself. In conventional systems, alarms often falsely convict the receiver card of failure.
U.S. Pat. No. 4,558,447 which issued Dec. 10, 1985 to Freeman et al., entitled "Self-testing Facilities of Off-chip Drivers for Processor and the Like" discloses a simple circuit which compares the input and output signals of an off-chip driver but requires the use of two clocks, clock A for the data and clock B for the test circuit. The added complexity of the second clock (clock B) introduces additional reliability issues and must further be adjusted to handle the variable delay which is introduced by the driver circuitry.
Also the Freeman design is incapable of handling functional, in-circuit at speed testing with off chip driver delays exceeding one clock period.